A whole onion does not start tears. The blade ruptures cells and mixes substances that had been kept in separate compartments. From that mix comes an irritating gas, not “onion water.”

The plant takes up sulfur from the soil and builds defense and flavour compounds. After cutting, alliinase enzymes meet sulfur precursors. An enzyme called lachrymatory-factor synthase steers the reaction toward propanethial S-oxide, a volatile compound described in plant biochemistry, including a 2002 Nature paper.

The gas reaches the moist surface of the eye. Sensory nerves signal irritation, and the tear glands dilute and wash it away. The story that “sulfuric acid forms in the eye” is a false shortcut: there is a specific tear factor and a protective reflex, not a drop of concentrated acid.

A sharp knife crushes less tissue. Ventilation carries some of the vapor away. Chilling the onion may slow evaporation, with uneven results. Sealed goggles isolate the eyes, even if they are awkward at the chopping board.

Cutting under a running tap limits vapor, but a slipping blade is a greater risk than a few tears. Tricks such as holding bread in the mouth do not stop the chemistry.

The tearing is usually brief and harmless — a plant defense meeting a human reflex. Severe irritation or an eye injury is outside this article; that needs medical advice.

Image: Gerrit Dou, Girl Chopping Onions / Wikimedia Commons. A period painting, not a laboratory photograph. Cropped to 16:9.

Source consulted: Why do onions make you cry? | American Chemical Society; An onion enzyme that makes the eyes water | Nature.